1. Field of the Invention
This invention relates to a semiconductor laser device suitable as a light source for optical communications and a method of producing the same. In particular, this invention relates to a semiconductor laser device which can emit laser light oscillating stably in a single fundamental transverse mode at low threshold currents, and a method of producing the same.
2. Description of the Prior Art
Semiconductor laser devices which can emit laser light oscillating in the 1 .mu.m wavelength band region (1.1 to 1.7 .mu.m) are currently the subject of intensive research with a view to utilization as light sources for optical communications (high-speed long-distance communications). This is due to the fact that the propagation loss in the quartz fibers used for optical communications is extremely small in this wavelength band. In particular, in low-loss quartz fibers manufactured from high-purity materials, no appreciable material dispersion occurs in the 1.3 .mu.m wavelength, and therefore, by using a semiconductor laser which emits a light having a wavelength in this band, cutoff frequencies exceeding 1 GHz.multidot.km can be realized.
FIG. 35 shows a cross-sectional view of a conventional semiconductor laser device of the type used as a 1.3 .mu.m light source for optical communications. This semiconductor laser device has a buried heterostructure (BH structure).
As shown in FIG. 35, this device comprises a mesa stripe structure in which an n-type indium phosphide cladding layer 22, an undoped InGaAsP active layer 23, a p-type indium phosphide cladding layer 24, and a p-type InGaAsP cap layer 25 is formed upon the semiconductor substrate 21. A burying layer which comprises a p-type indium phosphide current blocking layer 26 and an n-type indium phosphide current blocking layer 27 covers the sides of the mesa stripe structure. The device has an AuZn electrode 28 is formed on a p-type InGaAsP cap layer 25 and an AuGe electrode 29 on the reverse face of the substrate 21.
In the semiconductor laser device illustrated in FIG. 35, the sides of the mesa stripe structure including the active layer 23 are covered by the burying layer, and consequently laser light in a single fundamental transverse mode is obtained. Also, the structure is such that, if a voltage with a forward bias is applied on the mesa stripe structure, then a voltage with a reverse bias is applied upon the p-n junction within the burying layer. Consequently, currents flowing through the burying layer are diminished, and injection currents are confined within the mesa stripe structure. Therefore, the threshold current level is reduced, and stable fundamental transverse oscillation at a low current can be realized.
Next, a method for the production of the semiconductor laser device illustrated in FIG. 35 will be explained. First, using a liquid phase epitaxy (LPE) technique, the n-type indium phosphide cladding layer 22, the undoped InGaAsP active layer 23, the p-type indium phosphide cladding layer 24, and the p-type InGaAsP cap layer 25 are grown, in that order, upon the substrate 21.
Next, in order to form the portion into which the burying layer is to be buried, an etching mask with a mesa stripe pattern of the prescribed width is formed on the p-type InGaAsP cap layer 25, after which the p-type InGaAsP cap layer, p-type indium phosphide cladding layer 24, undoped InGaAsP active layer 23, and n-type indium phosphide cladding layer 22 below the region not covered by the etching mask are etched. In this manner, the mesa stripe structure composed of the indium phosphide cladding layer 22, the InGaAsP active layer 23, an indium phosphide cladding layer 24, and the p-type InGaAsP cap layer 25 is formed over the substrate 21. The pattern of the etching mask defines the width of the active layer 23.
Next, using an LPE technique, the p-type indium phosphide current blocking layer 26 and the n-type indium phosphide current blocking layer 27 are successively grown, in that order from the substrate side, on the region of the substrate 21 above which the n-type indium phosphide cladding layer 22, the undoped InGaAsP active layer 23, the p-type indium phosphide cladding layer 24, and the p-type InGaAsP cap layer have been removed by the etching process. This is performed in such a manner that the burying layer covers the sides of the mesa stripe structure, and, moreover, so that the height of the interface (p-n junction) between the p-type indium phosphide current blocking layer 26 and the n-type indium phosphide current blocking layer 27 is the same as or slightly greater than the height of the active layer 23.
After removal of the etching mask, the AuZn electrode 28 is formed above the p-type InGaAsP cap layer 25 and the n-type indium phosphide current blocking layer 27, and the AuGe electrode 29 is formed on the reverse face of the substrate 21.
In the conventional semiconductor laser illustrated in FIG. 35, the formation of an active layer 23 with the prescribed stripe width necessitates a process whereby, after a multilayer containing the active layer 23 has been formed over the entire substrate 21, a prescribed region of this multilayer is etched with high precision to form a mesa stripe structure with the required width. In order to stabilize the transverse mode oscillation of the laser, the deviation of the width of the active layer 23 from the prescribed value must not exceed the order of 0.1 .mu.m. However, in the above-described conventional process, the mesa stripe structure is formed by deep etching of a specified portion of the multilayer formed over the entire wafer, and therefore the determination of the width of the active layer 23 with good reproducibility at a precision of this order is extremely difficult. Consequently, a large statistical dispersion arises in the width of the active layer 23.
Moreover, in order to reduce the thresholed current level, the thickness of the various layers must be appropriately controlled so that the height of the p-n junction in the burying layer approximately coincides with the height of the active layer 23. If the height of the p-n junction deviates excessively from the height of the active layer 23, then the currents flowing through the burying layer rather than the active layer 23 increase, and consequently the threshold current level rises.
Also, since etching and other processes are necessary between the crystal growth process for the formation of the mesa stripe structure and the crystal growth process for formation of the burying layer, the overall process is complex. Furthermore, during the period between the two crystal growth processes, impurities may enter the crystal layers from the ambient atmosphere.
FIG. 36 shows another type of conventional semiconductor laser device (Japanese Laid-Open Patent Publication No. 64-25590). This semiconductor laser device comprises a ridge 32 of width 5 .mu.m and height 2.2 .mu.m formed along the &lt;011&gt; direction (direction of the resonator, perpendicular to the plane of the Figure) on the (100)-oriented n-type gallium arsenide substrate 31. Also, the device comprises a triangular mesa stripe structure on the ridge 32. The mesa structure has an n-type gallium arsenide buffer layer 33, an n-type aluminum gallium arsenide cladding layer 34, an undoped GaAs active layer 35, and a p-type aluminum gallium arsenide cladding layer 36 which are laminated, in that order, upon the substrate 31. The side faces of this structure are (111) B facets, and the angle formed between the facet and the surface of the substrate 31 is 54.7.degree.. The side face of the mesa stripe structure constitutes a facet with a specified crystal plane orientation in this manner because the various layers are grown under conditions such that crystal growth on the (111) B plane is difficult.
The device has a multilayer similar to the mesa stripe structure upon the region of the substrate 31 other than that upon which the mesa stripe structure is formed. In addition, the device comprises an n-type aluminum gallium arsenide barrier layer 41, a p-type aluminum gallium arsenide cladding layer 42, and a p-type GaAs contact layer 43 which are formed in that order from the substrate side, and a p-terminal electrode 44 formed over the p-type GaAs contact layer 43 and an n-terminal electrode 45 formed over the reverse face of the substrate 31.
A method for the production of the laser device has the advantage of not requiring an etching process between the crystal growth process for formation of the mesa stripe multilayer structure and the crystal growth process for formation of the burying layer. Also, since the two kinds of crystal growth processes can be performed continuously as a single-step process, the possibility for entry of impurities from the surrounding air into the crystal layers is diminished.
However, in this method, the ridge 32 is formed by deep etching of the substrate 31, and therefore the formation of the ridge 32 with the prescribed width and with a good yield is difficult, which constitutes a problem.
Moreover, since a double heterostructure is formed above the ridge 32, a large level difference exists on the wafer before the formation of the burying layer, and therefore the levelling of the wafer surface by the formation of the burying layer is difficult. If the surface of the wafer is not flat, then, if the semiconductor device is to be mounted on a material such as a heat sink, the proper contact between the heat sink, etc., and the upper surface of the wafer cannot be effected, which constitutes another problem.
Furthermore, in this conventional technology, the thickness of the various layers must be precisely controlled so as to ensure that the height of the p-n junction in the burying layer coincides with the height of the active layer, and consequently the problem of low manufacturing yield also arises.
FIG. 37 shows another type of conventional semiconductor laser device. This semiconductor laser device has a multilayer structure of the same type as that of the mesa stripe structure of FIG. 35 along the &lt;011&gt; direction (direction of resonator, perpendicular to plane of the Figure) over a (100)-oriented n-type indium phosphide substrate 51.
This multilayer structure is formed by depositing a silica film, having a rectangular opening with the long side along the &lt;011&gt; direction, on the n-type indium phosphide substrate 51, after which, using an MOCVD technique, an n-type gallium arsenide buffer layer 52, an n-type aluminum gallium arsenide cladding layer 53, an undoped GaAs active layer 54, and a p-type aluminum gallium arsenide cladding layer 55 are successively formed by selective growth, in that order, within the rectangular opening only. As in the preceding method of fabrication, the side faces of the multilayer structure formed in this manner are (111) B facets. The crystal growth does not occur on the SiO.sub.2 film. After the selective crystal growth process, the SiO.sub.2 film is removed from the substrate 51 by etching.
Then, using an LPE technique, a p-type aluminum gallium arsenide burying layer 56, an n-type aluminum gallium arsenide burying layer 57, a p-type aluminum gallium arsenide burying layer 58, and a p-type GaAs cap layer 59 are successively formed, in that order, over the substrate 51 from which the SiO.sub.2 film has been removed. The surface of the wafer is levelled by the growth of these layers. In order to reduce currents flowing in the burying layers, the height of the interface (p-n junction surface) between the p-type aluminum gallium arsenide burying layer 56 and the n-type aluminum gallium arsenide burying layer 57 is controlled so as to nearly coincide with the height of the active layer 54.
Also, a p-terminal electrode 60 is formed over the p-type InGaAsP cap layer 59 and an n-terminal electrode 61 over the reverse face of the substrate 51.
In this method of fabrication, no ridge is formed on the top surface of the substrate 51, therefore, the level difference on the wafer surface prior to the formation of the burying layers is comparatively small, which tends to facilitate the levelling of the wafer surface by the formation of the burying layers.
However, the precise control of thickness of the burying layers so as to ensure that the p-n junction in the burying layers is formed in the vicinity of the active layer 54 is difficult, and consequently the production yield is low.
Moreover, when the SiO.sub.2 film is removed from the substrate 51 by a hydrofluoric acid or other type of etching fluid, concomitant etching damage to the mesa stripe multilayer structure also constitutes a problem. This etching damage reduces the production yield and reliability of the semiconductor laser device.
The above-described semiconductor laser has other problems stated below. In optical instruments employing coherent light, the polarization of the coherent light possesses important significance. In general, the light emitted from semiconductor laser devices is polarized in a TE wave, and the direction of polarization is parallel to the active layer. Therefore, two semiconductor laser devices are required in order to obtain two varieties of laser light with different directions of polarization. For example, if two semiconductor laser devices are arrayed in series, a half-wave plate can be used to rotate the direction of polarization of the light emitted by one of the semiconductor laser devices relative to that of the light emitted by the other, thereby obtaining two laser beams with mutually distinct directions of polarization. However, this conventional technique requires two semiconductor laser devices as well as optical components such as half-wave plates, etc., therefore, a larger number of components is required, which is disadvantageous as regards miniaturization of devices incorporating such devices.
The type of semiconductor laser device illustrated by FIG. 35 has the problem of increased leakage current flowing across the p-n junction of the burying layer concomitantly with elevations in the temperature of the device. Consequently, the driving current required for the desired light output increases when the temperature of device rises. Also, owing to the strong temperature dependence of the current-light output characteristic and oscillation frequency, the temperature characteristics of the semiconductor laser device deteriorate and the reliability decreases.
Furthermore, another problem arises from the fact that the response characteristic of the light output with respect to the modulation of the driving voltage deteriorates due to the formation of junction capacitance in the vicinity of the p-n junction.
In the following, the various problems arising from the burying layers of other conventional burying type semiconductor laser devices will be explained with reference to FIGS. 38 and 39. Both of these semiconductor laser devices have a mesa stripe structure formed by successively growing an n-type cladding layer 72, an active layer 73, a p-type cladding layer 74, and a p-type cap layer 75, in that order, upon an n-type substrate 71. These two devices are characterized by their burying layers.
First, the semiconductor laser device illustrated in FIG. 38 will be described. The burying layer of this semiconductor laser device is a semi-insulating semiconductor layer 81.
The upper surface of the wafer is levelled, and on the levelled upper surface is formed a p-terminal electrode 77. Also, an n-terminal electrode 78 is formed on the reverse side of the substrate.
This semi-insulating semiconductor layer 81 is composed of an epitaxially grown semiconductor layer doped with transitional metals such as chromium, iron or cobalt, etc., as impurities. These impurities form deep-level traps in the semi-insulating semiconductor layer 81. Owing to the trapping by these deep levels of carriers injected into the semi-insulating semiconductor layer 81, almost no current flows within this semi-insulating semiconductor layer 81. Therefore, when the voltage which drives this semiconductor laser device is applied upon the electrodes, the flow of currents injected from the electrodes 77 and 78 is confined in the mesa stripe structure.
However, in the semiconductor laser device of FIG. 38, the following problems arise from the doping of the semiconductor layer with transition metals such as chromium, iron or cobalt as impurities.
If the semi-insulating semiconductor layer 81 (which constitutes the burying layer) is grown by liquid phase epitaxy, then, since the density of deep-level traps formed in the semi-insulating semiconductor layer 81 grown in this manner is low, consequently, carriers injected into the burying layer cannot be adequately trapped, therefore, the insulating effect manifested by the burying layer is inadequate. The low density of deep-level impurities is due to the poor solubility of the transition metals in the melt which is used for an LPE.
On the other hand, if the semi-insulating semiconductor layer 81 is formed by vapor phase growth, then the deep groves between the several mesa stripe structures formed on the substrate 71 prior to subdivision into chips cannot be adequately filled by the semi-insulating semiconductor layer 81.
Moreover, another problem arising in the semiconductor laser device of FIG. 38 is the diffusion of transition metals into the active layer 73 or cladding layer 74 of the mesa stripe structure from the burying layer when the temperature of the latter has been elevated by heat generated during the operation of the said semiconductor laser device. When transition metals diffuse into the active layer 73, the luminous efficiency of the semiconductor laser device drops and the operating life is shortened.
Next, the semiconductor laser device illustrated in FIG. 39 will be described. This semiconductor laser device has a burying layer composed of polyimide resin 91. A thin p-type epitaxially grown layer 92 and a silica layer 93, which serve to protect the semiconductor layers in the mesa stripe structure, are interposed between the polyimide layer 91 and the substrate 71.
The burying layer, being composed of polyimide resin 91, is an electrical insulator. Therefore, when the voltage which drives this semiconductor laser device is applied to the electrodes 77 and 78, the resulting current does not flow through the burying layer. Consequently, the current injected from the electrodes 77 and 78 is confined in the mesa stripe structure.
However, the semiconductor laser device of FIG. 39 involves a problem in that the thermal conductivity of the polyimide layer 91 is low. If the thermal conductivity of the burying layer is low, then the heat generated within the active layer 73 during operation is not adequately conducted to the exterior of the device, hence, the temperature of the said active layer 73 rises. Concomitantly with this temperature elevation of the active layer 73, the threshold current increases, and the differential efficiency drops.
Also, the width of the mesa stripe structure of each of the semiconductor laser devices is of the order of 2 .mu.m. That is, in order to ensure that the transverse mode of the emitted laser light is of the single fundamental transverse mode, a narrow mesa stripe structure of the order of 2 .mu.m in width must be formed on the substrate. Since each of these devices has a narrow mesa stripe structure of the order of 2 .mu.m in width, the mesa stripe structure is easily damaged when the wafer surface is levelled. Owing to this problem, the production yield tends to be low. Moreover, if the width of the mesa stripe structure is greater than the prescribed value, then the semiconductor laser device becomes incapable of stable oscillation in the fundamental transverse mode.
Distributed feedback (DFB) semiconductor laser devices, in which a periodic variation in refractive index is imparted to the current injection site, are currently drawing attention as light sources for communications and optical measurement by virtue of easy oscillation in the single longitudinal mode and stability of oscillatory wavelength even under variations of temperature and driving current. Since high reliability and low driving current are required in this field of application, BH semiconductor laser devices, which oscillate in the fundamental transverse mode at a low-current threshold level, are the principal types used in this field.
FIG. 40 shows an example of a conventional DFB semiconductor laser device with a BH structure. This type of DFB semiconductor laser device is fabricated in the following manner. First, an n-type AlGaAs first cladding layer 732, an undoped gallium arsenide active layer 733, a p-type AlGaAs carrier barrier layer 734, and a p-type AlGaAs guide layer 735 are successively grown, in that order, over the n-type gallium arsenide substrate 731. Next, a diffraction grating is ruled on the upper surface of the p-type AlGaAs guide layer 735. Then, a p-type AlGaAs second cladding layer 736 and a p.sup.+ -GaAs cap layer 737 are grown on the p-type AlGaAs guide layer 735 upon which the diffraction grating has been ruled. The surface of the p-type AlGaAs second cladding layer would oxidize if exposed, which would impede the subsequent growth processes, and therefore the p.sup.+ -GaAs cap layer 737 is formed in order to prevent this surface oxidation. Next, two parallel grooves extending down to the substrate are formed by etching, thereby forming a stripe-like mesa. Then, an n-type AlGaAs first current blocking layer 738, a p-type AlGaAs second current blocking layer 739, and an n-type AlGaAs third current blocking layer 701 are successively grown, thereby filling the grooves. Finally, a p.sup.+ -GaAs contact layer 702 is grown so as to level the surface, after which a p-terminal electrode 703 on the uppersurface and an n-terminal electrode 704 on the reverse side of the substrate are formed.
In order to fabricate the BH type of DFB semiconductor laser device shown in FIG. 40, after growing the p.sup.+ -GaAs cap layer 737, two parallel grooves must be formed to form the stripe-like mesa. Also, in order to obtained stable single transverse mode oscillations, this etching must be controlled with sufficient accuracy as to ensure the stripe width of the mesa is in the range 1 to 1.5 .mu.m. Moreover, the several current blocking layers must be grown to the desired thickness on the two sides of the stripe-like mesa.
Thus, like other prior art described above, the fabrication of this DFB semiconductor laser device also requires extremely precise etching processes as well as processes for the growth of current blocking layers with highly accurate control, therefore, the number of steps in the overall fabrication process is large. Consequently, these conventional methods of fabrication are poorly suited for mass production. Furthermore, since the characteristics of the devices depend upon the etching precision, shortcomings arise in that performance is necessarily unstable and the manufacturing yield is low.